Organic electronic devices employing organic materials are expected to be developed for a wide range of basic elements and uses including organic electroluminescence elements (hereunder, “organic EL elements”), organic transistors and organic solar cells. In addition, “organic devices”, in the wider sense of having a hole injecting transporting layer, include quantum dot light emitting elements in which organic EL luminescent dopants are replaced with inorganic semiconductor nanoparticles.
Organic EL elements are charge injecting-type self-luminous devices utilizing luminescence produced during recombination of electrons and holes that have reached the luminescent layer.
The element structure of an organic EL element is cathode/luminescent layer/anode. In order to obtain high luminous efficiency in an organic EL element it is necessary to efficiently and rapidly supply electrical charges (holes and electrons) to the luminescent material acting as the luminescent center, but due to a high energy barrier between the anode or cathode and the organic functional layer, such as the luminescent layer, injection of electrical charges is not easily accomplished. For this reason, the energy barrier between the electrode and organic functional layer has conventionally been lowered by optimizing the work function of the anode or cathode, which is accomplished by adding a charge transport material to the luminescent layer, forming a hole transporting layer between the anode and the luminescent layer, or forming an electron transporting layer between the cathode and the luminescent layer. In the original organic EL elements, therefore, the organic functional layer containing the luminescent layer usually had a two-layer structure comprising a luminescent layer and a hole injecting layer, or a three-layer structure comprising an electron transporting layer, a luminescent layer and a hole transporting layer.
Several multilayer structures have recently been proposed, including the 5-layered structure: electron injecting layer/electron transporting layer/luminescent layer/hole transporting layer/hole injecting layer, with the aim of obtaining high luminous efficiency and long lifetime.
These layers other than luminescent layers, such as the electron injecting layer, electron transporting layer, hole transporting layer and hole injecting layer, are considered to have effects of facilitating injecting and transport of electrical charge into the luminescent layer, or effects of blocking to maintain balance between the electron current and hole current, or effects of preventing diffusion of light energy excitons.
In addition to organic EL elements, organic solar cells may be mentioned as examples of organic electronic devices using organic materials with a certain level of carrier mobility.
The most basic structure of an organic solar cell is one wherein an organic thin-film with the same two-layer structure as an organic EL element is sandwiched between electrodes. The photocurrent produced by absorption of light into the organic thin-film can be utilized to obtain electromotive force. The current that flows may be considered to be flow of the carrier produced by light utilizing the carrier mobility of the organic material. If the charge injecting barrier between the organic material and the electrode can be reduced, it is possible to obtain more efficient electromotive force. This is, in a sense, a mechanism opposite of that of an organic EL element.
An organic transistor is another example of an organic electronic device. An organic transistor is a thin-film transistor that uses an organic semiconductor material composed of a n-conjugated organic high molecular or organic low molecular compound in the channel region. Common organic transistors comprise a substrate, a gate electrode, a gate insulating layer, a source/drain electrode and an organic semiconductor layer. In an organic transistor, the voltage applied to the gate electrode (gate voltage) is varied to control the charge at the interface between the gate insulating film and organic semiconductor film, and the current value between the source electrode and drain electrode is varied for switching.
When organic semiconductor materials employed in such organic transistors are used, however, the charge injecting barrier with the source electrode or drain electrode has been large, causing problems with element driving. It is expected that reducing the charge injecting barrier between the organic semiconductor layer and the source electrode or drain electrode will improve the on-state current value for organic transistors and stabilize element characteristics.
On the other hand, in regard to methods for producing organic electronic devices involving formation of luminescent layers or charge transport layers, there have been proposed methods for producing organic electronic devices such as organic EL elements having a luminescent layer or charge transport layer formed by a vapor deposition method such as vacuum vapor deposition or ion sputtering, or a coating method in which an organic material having luminescence or a charge transport property is dissolved, dispersed or mixed with a solvent and applied onto a substrate to form a coating film (spin coating, printing, ink-jet methods and the like).
For production of organic electronic devices, coating methods, involving application onto a substrate with or without a solvent, have advantages over vapor deposition methods such as vacuum vapor deposition, in that they do not require large vapor deposition apparatuses and allow the fabrication process to be simplified, while also having high material utilization efficiency and low cost, and permitting large-area substrates to be processed. In addition, because materials can be separately applied in parallel, such as for RGB in organic EL elements, there are significant advantages to forming organic electronic devices by coating methods.
Anions such as sulfate ion, or cations, have been removed in some cases (Patent document 1) for the purpose of extending the life of polythiophenesulfonic acid (PEDOT/PSS), as a hole injecting material with satisfactory film formability by coating and high charge transporting capacity and charge injecting capacity.
Strategies aimed at obtaining a hole injecting layer with satisfactory injecting properties and electrical charge mobility include creating a hole injecting layer by forming a thin-film by a vapor deposition method using a transition metal oxide such as vanadium pentaoxide or molybdenum trioxide (Patent documents 2-4), creating a hole injecting layer by forming a mixed film by covapor deposition with molybdenum trioxide (MoO3) and an amine-based low molecular compound (NPD) (Patent documents 5 and 6), and forming a hole injecting layer by pulverizing molybdenum trioxide to form fine particles and dispersing them in a solution to form a slurry, which is then coated (Patent document 7).
In addition, for organic luminescent devices other than organic EL elements, such as light-emitting electrochemical cells (LECs) and electrochemiluminescence (ECL) (for example, Non-patent document 1), as well as for organic electronic devices such as organic transistors or organic solar cells, wherein a known strategy is to increase the carrier density in the organic transistor layer near the electrode by introducing a charge-transfer complex into the organic transistor, as an attempt to improve the on-state current value in the organic transistor and stabilize the element characteristics by lowering the charge injecting barrier between the organic transistor layer and the source electrode or drain electrode in the organic transistor (Patent document 8, for example), it is important for the charge injecting barrier between the electrode and organic layer to be lowered for high charge injection efficiency, while it is also important to allow convenient and efficient production by coating methods.